The present invention generally relates to magnetic recording media which are often loaded into magnetic disk apparatuses, magnetic tape apparatuses and the like, and which are used as external memory devices of a computer and to methods of producing such magnetic recording media. More particularly, the present invention relates to a perpendicular magnetic recording medium which has a double-layered structure and a method of producing such a perpendicular magnetic recording medium.
Recently, the magnetic disk apparatuses are made compact and the magnetic recording and reproduction are carried out with a high density. As a high density recording system which is far superior to the conventional longitudinal magnetic recording system, there is the so-called perpendicular magnetic recording system. According to the perpendicular magnetic recording system, a residual magnetization is formed in a direction perpendicular to the surface of the magnetic recording medium, and there is active research and development of perpendicular magnetic recording in many research institutes and companies.
A single pole head and a ring head have been proposed for the perpendicular magnetic recording. On the other hand, perpendicular magnetic recording media having, variously a single-layer structure and a double-layered structure have been proposed for the perpendicular magnetic recording. The single-layer recording medium has a recording layer made of CoCr, for example. The double-layered recording medium has a recording layer and a high permeability layer which is provided under the recording layer and is made of NiFe, for example. When carrying out the perpendicular magnetic recording, it is known that optimum results are obtained by use of a combination of the single pole head and the double-layered recording medium.
FIG. 1 shows a double-layered perpendicular magnetic recording medium 1 and a single pole head 2. The recording medium 1 includes an Al substrate 3 which is subjected to an alumite surface processing, a NiFe soft magnetic under layer 4, and a CoCr perpendicular magnetic recording layer 5 which has a perpendicular magnetic anisotropy. The head 2 includes a main pole 6, an auxiliary yoke 7, and a coil 7a. The under layer 4 is often referred to as a backing layer. The under layer 4 functions as a path for the magnetic flux, and the recording and reproducing sensitivity is improved by the provision of the under layer 4.
When a stray magnetic field source 8 exists when carrying out the perpendicular magnetic recording and reproduction using the arrangement shown in FIG. 1, a magnetic flux 9 from the source 8 is concentrated at the main pole 6, and in addition, the concentrated magnetic flux 9 easily flows to the under layer 4 via the recording layer 5 as indicated by an arrow 10. Because of this flow of the magnetic flux 9, there is a problem in that the information recorded on the recording medium 1 is easily erased and it is a main objective in the perpendicular magnetic recording and reproduction to solve this problem.
Accordingly, when reducing the perpendicular magnetic recording system to practice, it is highly desirable to develop a perpendicular magnetic recording medium in which the erasure of information due to the stray magnetic field is suppressed.
FIG. 2 shows a conventional perpendicular magnetic recording medium 1A. In FIG. 2, those parts which are essentially the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. A soft magnetic under layer 4A includes magnetic domains 11-1, 11-2, 11-3 and the like as shown in FIG. 2 and shown on an enlarged scale in FIG. 3. An arrow 12 indicates a direction of the magnetic spin. In the magnetic domains 11-1, 11-2 and the like, the magnetic spin is parallel to the surface of the under layer 4A. In addition, a domain wall 13 clearly exists between the magnetic domains. The size of the magnetic domain is in the order of several .mu.m to several mm. Generally, the under layer 4A has a thickness in the range of 0.5 .mu.m, and in order to improve the recording and reproducing sensitivity, the magnetic characteristics are set such that the permeability is 1500 or greater and the coercivity of 10 Oe or less. On the other hand, the recording layer 5 usually has a thickness in the range of 0.2 .mu.m, and the perpendicular coercivity is set to approximately 1000 Oe.
FIG. 4 shows a waveform of a signal which is reproduced from one track turn of the recording medium 1A when a signal having a constant level is recorded. FIG. 5 shows a waveform of a signal which is reproduced from the same track turn when a magnetic field of 3 Oe is applied to the recording medium 1A from directly above the head 2 during the reproduction. The recording magnetic field of the head 2 at the time of the recording is set to be greater than the perpendicular coercivity of the recording layer 5, and is normally 2000 Oe or greater. Accordingly, the external magnetic field of 3 Oe is considerably small compared to the recording magnetic field of the head 2.
However, the signal level of the reproduced signal decreases at parts as indicated by 14 and 15 in FIG. 5 even in response to the small external magnetic field. This decrease in the signal level is caused by the fact that the recorded information on the recording layer 5 is partially erased by the external magnetic field. Normally, a stray magnetic field in the range of 3 to 5 Oe inevitably exists within the magnetic recording apparatus. As a result, the erasure of recorded information due to the stray magnetic field easily occurs with the conventional recording medium 1A, and the reliability of the recorded information is poor.
It may be regarded that the erasure of the recorded information due to the stray magnetic field is closely related to the random motion of the domain walls in the under layer 4A. In other words, it may be regarded that the permeability, at a part of the under layer 4A, temporarily becomes high when the domain walls of the under layer at this part under go a random motion due to the stray magnetic field, and the information recorded at a corresponding part of the recording layer 5 is erased because of the magnetic flux of the stray magnetic field flowing to the part of the under layer 4A via the main pole 6 of the head 2.